International Publication Number WO 2012/103629 A1, Anaerobic Fermentation to Produce Biogas, describes a process and apparatus for the anaerobic digestion of organic wastes, preferably to also produce a useful biogas. The waste may have a total solids (TS) concentration of 6% or less while a digester is operated at a higher solids concentration, for example with a feed TS concentration of 8-12%. One or more separation stages downstream of the digester separate active bacteria and undigested organics from the digestate, and return separated matter to the digester. Optionally, a feed thickening apparatus and step may be provided upstream of the digester. The upstream thickener and recycle from the downstream separation stages are operated such that the TS of the combined inputs to the digester is within a desired range.